The causes and mechanisms of many autoimmune rheumatic diseases remain obscure, and therapy largely remains empirical and non-specific. The long-term goal of this research program is to define mechanisms in these diseases as a way to impact disease diagnosis, therapy, prediction and prevention. Recent studies have demonstrated that autoantigens in myositis are expressed at high levels in inflamed muscle, particularly in regenerating muscle cells. These studies have suggested that (i) regenerating cells may provide a critical source of autoantigen, and (ii) immune-mediated cell damage may be focused on such regenerating cells, inducing antigen release that drives the immune response, and augmenting antigen expression through stimulating more regeneration. Preliminary studies demonstrate that autoantibodies from myositis patients specifically recognize previously undefined autoantigens expressed exclusively in myoblasts or differentiating muscle cells, and that cytotoxic lymphocytes cluster around such cells in myositis muscle. This application will identify these novel autoantigens, define their expression patterns in polymyositis and dermatomyositis and whether antigen expression and immune response are related in individual patients (indicating that antigen expression may shape selection of immune targets). This application will also seek to provide evidence in vivo that cells expressing such muscle- and differentiation-specific antigens are the preferential focus of immune effector pathways in myositis. Furthermore, since most of these autoantigens are substrates for granzyme B, these studies will define whether such fragments are present in situ at sites of cytotoxicity. Such data would provide important information regarding the activity and role of the granzyme pathway in rheumatic diseases. These goals will be achieved by pursuing the following specific aims: (1) Use differentiating myoblasts as a source of autoantigens to identify novel phenotype-specific autoantibodies in myositis patients, and define their diagnostic sensitivity and specificity; (2) Define the relationship between autoantibody response and antigen expression in vivo; and, (3) Define the cell type and differentiation state targeted by immune effector pathways in myositis biopsies. These studies will define the contents of distinct muscle cell types and differentiation states as targets of the immune response, and the cells expressing such antigens as the focus of immune effector pathways in autoimmune myositis. Proving that antigen source and specific immune effector pathways are interacting at the site of disease propagation in vivo will highlight components of a feedforward loop that may be therapeutically tractable (e.g. antigen expression, granzyme B inhibition). In addition to pathogenic insights, the studies will also result in the development of new diagnostic assays for myositis and potentially other autoimmune rheumatic diseases .Narrative: These studies will investigate mechanisms in autoimmune rheumatic diseases to try to design new markers for diagnosis, and pathways that might be targets of new therapies. In particular, we will focus on the cells accomplishing repair of damaged tissues in autoimmunity, which appear, themselves to be targets of immune damage. We will define the molecules recognized in repairing cells, and how these may participate in ongoing damage.